Toners or developing agents in the form of finely divided particles for developing electrostatic latent images in electrophotography have been heretofore produced by a so-called crushing method. According to this method, a colorant such as carbon black, an electric charge controlling agent such as a certain dyestuff, and an anti-offset agent such as a wax are mixed and kneaded together with a melted thermoplastic resin, thereby to disperse them in the resin, cooling, crushing and pulverizing the resultant solid mixture with, for example, a jet mill, to powders of a desired particle size.
In this method, it is necessary that the resin used be brittle so that a mixture of the resin and the additives as mentioned above be readily crushed. However, when a resin used is too brittle, the resultant toner is excessively finely divided during the use in an electrophotographic apparatus, and contaminates the inside of the apparatus or forms fog on developed positive images. On the other hand, when a resin used is readily melted, the resultant toner is apt to aggregate together and is undesirably reduced in fluidity, but also there takes place filming on an photoconductive body to deteriorate quality of positive images.
It is also necessary that individual toner particles have colorants and charge controlling agents equally and finely dispersed therein, and be capable of being equally electrified so as to produce high quality positive images. However, according to the conventional crushing method, colorants and charge controlling agents are unequally divided among individual toner particles with varied particle sizes. Thus, it is inevitable that positive images have background contamination as well as fog thereon. The apparatus is also contaminated.
In particular, a charge controlling agents has an important effect upon copying performance of toners, but since the known charge controlling agents are in many cases 1-20 .mu.m in particle size, much time is needed to disperse the agent in a resin and thus productivity is low. Moreover, as a matter of fact, the agent can not be uniformly dispersed in a resin even after kneading over a long period of time.
As above set forth, the conventional crushing method has many disadvantages, and therefore there have been proposed in recent years many methods to produce toners directly by suspension or emulsion polymerization of a radical polymerizable monomer which contains colorants therein such as carbon black. In these methods, an oily monomer phase is polymerized in an aqueous phase containing a suspending agent dissolved therein such as polyvinyl alcohol. Accordingly, at least some portions of the suspending agent remain inevitably on the surface of the resultant polymer particles even after repeated washing, so that the polymer particles are very sensitive to humidity. Thus, such toners are low in triboelectricity under high humidity, and are apt to produce noncharged or reversely charged toners during the use, to provide a toner image with undesired fog or a toner image with an insufficient darkness.
It is an advantage of the toners produced by a conventional suspension or emulsion polymerization method that the toner is substantially spherical and has a high fluidity so that there is no need of adding a fluidizing agent such as silica to the toner. But, because of that sphericity, the toner is inferior in "blade cleanability".
In an electrostatic photography using plain paper as a substrate on which toner images are fixed, an latent image is formed on the surface of an photoconductive body to which electrostatic charges have been given, the latent image is developed by the toner to a toner image, and the toner image is transferred onto a substrate, and then the toner image is fixed thereon, to provide a copy. Therefore, it is necessary that the toner remaining on the photoconductive body is removed therefrom after the toner image has been transferred onto the substrate to copy in succession. As one of the methods for removing the toner remaining on the photoconductive body, a blade cleaning method is known according to which the toner is scraped off with a cleaning blade after the toner image has been transferred onto the substrate. The blade is formed of various elastomers, among which a polyurethane elastomer is most preferred from the standpoint of mechanical properties such as resistance to abrasion.
In such a blade cleaning method, spherical toner particles enter beneath the blade when the blade scrapes the photoconductive body and roll between the blade and the surface of photoconductive body, so that the toner remains on the photoconductive body after the cleaning of the body with the blade.
Thus, in the production of toner particles by suspension polymerization, there has been proposed a method in which spherical polymer particles are agitated in a suspension medium at a high rate before the completion of the polymerization so that the spherical polymer particles are deformed, as described in Japanese Patent Application Laid-open No. 62-266560. However, according to the method, the polymer particles are apt to aggregate to each other on account of unreacted monomers remaining in the reaction system or the deformed polymer particles are restored to their original spherical particles at relatively high temperatures where the polymer particles are readily deformed, on account of surface tension they possess. Namely, effective deformation of spherical polymer particles is not attained. Agitation of the polymer particles at small rates or at low temperatures also fails to effectively deform the spherical polymer particles, although the aggregation of the particles is restrained. Furthermore, the polymer particles produced by the suspension polymerization have rather a wide particle size distribution. Thus, large spherical particles might be readily deformed, but small particles are not, and accordingly there arises a wide distribution in degree of deformation. Accordingly, as a further defect of the above method, small spherical particles remain undeformed and such small spherical particles elude cleaning by a blade on the photoconductive body.
A further method of producing toners has been recently proposed in which finely divided particles are adhered and fixed onto the toner particles by a so-called impact method, as described in Japanese Patent Application Laid-open No. 62-128866. However, since toner particles have a significant size distribution, it is necessary that the finely divided particles are of not more than about 1 .mu.m so that they are successfully fixed on the individual toner particles according to this method. Little improvement in blade cleanability is attained with such toner particles having such fine particles forced thereon.
Meanwhile, there is disclosed a method of improving triboelectricity of toner particles in Japanese Patent Application Laid-open No. 62-140636 or No. 62-246075. In this methods, finely divided triboelectric or electroconductive particles are forcibly made to collide with the surface of toner particles at high velocity, or toner particles are softened in a hot air stream and such particles are adhered onto the surface of toner particles. This method is not applicable, however, to deformed toner particles since the deformed toner particles have a tendency to become spherical under the conditions employed. In conclusion, no method has hitherto been known which improves both triboelectricity and blade cleanability of toner particles.